Microwave acoustic surface wave mixer and method of fabrication

ABSTRACT

Power density curves of acoustic surface waves generated in piezoelectric substrate members by sum and difference electromagnetic input signals have been found to peak at discrete distances from the input transducer. An effective microwave mixer is provided by positioning an appropriately tuned output transducer on the substrate member at a point coinciding with the desired sum or difference acoustic signal power peak.

United States Patent Inventor Andrew J. Slobodnik, Jr. Lowell, Mass.

Appl. No. 24,743

Filed Apr. 1, 1970 Patented Jan. 11, 1972 The United States of Americaas represented by the Secretary of the United States Air Force AssigneeMICROWAVE ACOUSTIC SURFACE WAVE MIXER AND METHOD OF FABRICATION 5Claims, 9 Drawing Figs.

U.S. Cl 307/883, 325/442, 325/445 Int. Cl H03f 7/00 Field of Search307/883;

References Cited OTHER REFERENCES Lean et al., Applied Physics Letters,"1 July 1969, p. l0- 12.

Primary ExaminerRoy Lake Assistant Examiner- Darwin R. HostetterAttorneys- Harry A. Herbert, Jr. and Willard R. Matthews, Jr.

52 mu n MICROWAVE ACOUSTIC SURFACE WAVE MIXER AND METHOD OF FABRICATIONBACKGROUND OF THE INVENTION This invention relates to microwave acousticsurface wave devices and particularly to mixers and other devices thatutilize the nonlinear acoustic properties of crystalline substratemembers.

Volume or bulk wave acoustic devices such as acoustic delay lines, phaseshifters and directional couplers have been used in microwave systemsfor some time. Recently, in an attempt to reduce power requirements,considerable effort has been expanded to perfect various acousticsurface wave devices.

Microwave-frequency surface wave devices have several advantages overtheir volume-wave counterparts. Surface waves require only one opticallypolished surface, whereas volume waves require two surfaces which mustbe parallel to optical tolerances. The fabrication techniques forsurface wave transducers are the same as those used for integratedcircuits, so that a surface wave delay line could, for example, befabricated on a substrate together with a transistor amplifier. Theamplification of surface waves by means of their traveling waveinteraction with drifting carriers in semiconductors has severaladvantages over the corresponding amplification of volume waves. Thesurface wave is accessible along the entire surface, so that it ispossible to make contiguously tapped delay lines for suchsignal-processing functions as pulse expansion and compression. Magneticsurface waves on ferrimagnetic substrates have been found to benonreciprocal and this makes them of potential use for such devices asisolators, circulators, and phase shifters. Acoustic surface wavewaveguides and directional couplers have been fabricated for use atmegahertz frequencies. The width of acoustic waveguide components atmicrowave frequencies would be of the order of micrometers. Thus, thereexists the possibility of entire microwave acoustic-integrated circuitswhich could be as much as five orders of magnitude smaller than theirelectromagnetic equivalents.

The current state of the art of microwave acoustic surface wave devicesis reviewed in detail in the publication, The Generation and Propagationof Acoustic Surface Waves at Microwave Frequencies, by Paul H. Carr,IEEE Transactions on Microwave Theory and Techniques, Vol. MTI'17, No.11, Nov. 1969.

Acoustic surface wave devices are inherently small, lightweight andrugged, and are therefore particularly adapted to airborne and aerospaceapplications. With reference to air borne radar applications the use ofmicrowave frequencies is the only way to obtain the 0.5-1 GHZ.bandwidths necessary to improve range resolution. In addition, theoperation of signal processing devices at microwave frequencieseliminates the necessity for frequency down-conversion and subsequentup-conversion with its insertion loss, increased complexity, and theloss of phase information. There is, therefore, a current need forefficient, small, rugged, lightweight microwave acousticsignal-processing devices of all types. In particular, amplifiers,harmonic generators, limiters and mixers that have modest powerrequirements and that operate without external bias supplies are yetunavailable. The present invention is directed toward providing suchsystem components and toward teaching novel methods and techniques thatwill lead to entire acoustic signal-processing systems.

SUMMARY OF THE INVENTION The present invention is a microwave mixercomprising a piezoelectric substrate member having one polished surfacesuitable for the propagation of acoustic surface waves, input and outputtransducer positioned at a discrete distance on the acoustic wavepropagating surface and means for delivering to the input transducer twoelectromagnetic wave input signals of different frequencies. The presentinvention is based upon the discovery that an acoustic surface waveresponsive to a transducer electromagnetic sum or difference signal willexhibit a power density peak at some given distance from the inputtransducer. This novel concept has been implemented in the presentinvention by measuring and recording the power densities of acousticsurface waves generated in response to desired 'mixed electromagneticwave signals. An efficient microwave mixer can then be fabricated byaffixing an appropriately tuned output transducer to the propagationsurface of the substrate member at a point that coincides with theoccurrence of maximum power density of the acoustic surface wave.

It is a principal object of the invention to provide a new and improvedmicrowave acoustic surface wave mixer.

It is another object of the invention to provide a microwave acousticsurface wave mixer that is rugged, lightweight and small and adapted toairborne and aerospace uses.

It is another object of the invention to provide a microwave acousticsurface wave mixer that is more efficient and requires less power thancurrently available devices of its type.

These, together with other objects, advantages and features of theinvention, will become more apparent from the following detaileddescription when taken in conjunction with the illustrative embodimentsin the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is an orthogonal view of amicrowavefrequency acoustic surface wave delay line;

FIG. 1A is a side view of the delay line of FIG. 1 schematicallyillustrating acoustic surface waves propagating therealong;

FIG. 2 is a graph illustrating the growth and decay of surface wavefundamental and harmonics as a function of distance from the inputtransducer;

FIG. 3 illustrates a microwave acoustic surface wave harmonic generator;

FIG. 4 illustrates a microwave acoustic surface wave limiter;

FIG. 5 illustrates a microwave acoustic surface wave mixer;

FIG. 6 illustrates a microwave acoustic surface wave amplifier;

FIG. 7 is a graph of the power in the fundamental frequency acousticsurface wave as a function of microwave input power, illustrating thelimiting action of the limiter of FIG. 4; and

FIG. 8 is a graph showing the growth and decay of a sum frequencysurface wave caused by mixing two discrete input signals in the mixer ofFIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS.1 and 1A, there is illustrated thereby an acoustic surface wave devicecomprising substrate member 10, input transducer 11 and outputtransducer 14. Substrate member 10 can be any suitable crystalline mediaor piezoelectric material such as lithium niobate (LiN O or bariumsodium niobate (Ba NaNb O Input transducer 11 consists of interdigitalfingers 12 and 13 which may be affixed to the propagating surface 9 byprinted or integrated circuit techniques. Output transducer 14consisting of interdigital fingers l5 and 16 is similarly affixed tosurface 9. The operation of such a device when used as a delay line isillustrated by FIG. IA. The electromagnetic wave input produces anelectric field between the half-wavelength spaced lines of theinterdigitaltype transducer on the piezoelectric substrate. Thepiezoelectric effect produces a stress which propagates along thesurface in both directions, the two acoustic powers being equal bysymmetry. The surface wave propagating toward the output transducer isdetected by means of the piezoelectric effect. The wave propagating inthe opposite direction can be terminated by an acoustic absorber such aswax or tape (not shown).

Certain nonlinear acoustic properties of crystalline media have beendiscovered through investigation and measurement of the acoustic surfacewave power densities along the substrate propagation surface. FIG. 2contains various curves that illustrate this nonlinear phenomena. Curves18, 19 and 20 represent the power densities measured at variousdistances from the input transducer of the second, third and fourthharmonics respectively of a 905 MHz input signal. Curve 17 representsthe fundamental of the same input signal. An examination of the curvesof FIG. 2 readily reveals substantial interaction between the harmonicsand the fundamental of acoustic surface waves propagating along acrystalline substrate member. Of particular importance is the discretepoint, unique to each, at which each harmonic exhibits a power peak.Also of particular importance is the negative slope that occurs in thefundamental curve 17. It is apparent that at the point of maximuminteraction between harmonics and fundamental, and beyond for a givendistance, the harmonics recontribute energy to the fundamental. Thecharacteristic peaks of the several harmonics and the unique negativeslope of the fundamental are phenomenon upon which the various noveldevices of the present invention are based.

Measurement of the power densities of acoustic surface waves can beaccomplished in any conventional manner. Laser'light deflectionconstitutes a particularly convenient tool for accomplishing these ends.Such a technique is well known in the prior art and has been used todetect and study the nonlinear effects of harmonic generation and mixingin surface wave delay lines. In practicing the technique a microwaveelectromagnetic wave is converted to an acoustic surface wave by meansof the interdigital input transducer. This wave periodically modulatesthe substrate surface which, according to well-known electromagneticscatter theory deflects any incident light into side lobes as well asinto the specular direction. The intensity of the deflected light isdirectly related to the intensity of the surface wave and the angulardirections are given by the grating equation:

Here A is the wavelength of the incident light and A is the surfacewavelength. The fundamental surface wave or any of its harmonics canthen be monitored by placing a suitable light detector at an anglecorresponding to the proper wavelength.

Using these procedures, nonlinear effects can be investigated as afunction of distance by scanning the laser along the direction ofpropagation of the surface wave as a function of input power. Thismeasuring technique is described in detail in the periodical article,Microwave Frequency Acoustic Surface Wave Propagation Losses in LiN Q-by A. J. Slobodnik, Jr., published in Applied Physics Letters,Volume 14,No. 3, page 94, Feb. 1, 1969.

FIG. 3 illustrates, schematically, a microwave harmonic generator of thetype comprehended by the present invention. It comprises a substratemember 21 having a surface 50 polished to accommodate the propagation ofacoustic surface waves. Substrate member 21 can be of lithium niobate orany other suitable crystalline media capable of supporting acoustic wavepropagation. An electromagnetic wave to acoustic surface wave inputtransducer 23 is affixed to surface 50 by printed circuit or integratedcircuit techniques. Input transducer 23 comprises interdigital fingers26 and 27 that are spaced to accommodate the desired operating frequencyof the device D,=(A/2). Acoustic surface wave to electromagnetic waveoutput transducer 22 is also affixed to surface 50. This outputtransducer consists of interdigital fingers 24 and 25 which are spacedto accommodate the desired harmonic. FIG. 3 is a harmonic generatordesigned to operate at the second harmonic requiring that spacing ofinterdigital fingers be equal to quarter wavelengths D,,=(A/4). Thedistance between input and output transducer L is determined by thesurface wave power peak of the desired harmonic. By way of example, theparameters of second, third and fourth harmonic generators fabricated inaccordance with the curves 18, 19 and 20 of FIG. 2 for an operatingfrequency of 905 MHz. would be: Second harmonic generator D =2D =1.93,u.m., L==3.4 mm. Third harmonic generator D,=3D,=l.93 ,um.,

L=2.6 mm. Fourth harmonic generator D,=4D,,=l.93 ,um., L=2.5 mm.

A microwave acoustic surface wave limiter which utilizes the novelconcepts of the present invention is schematically illustrated by FIG.4. The limiter comprises a substrate member 28 of piezoelectricmaterial, input transducer 32 and output transducer 29. Surface 51 ofthe substrate member is polished to accommodate the propagation ofacoustic surface waves. Input transducer 32 consists of interdigitalfingers 33 and 34 which are fixed to the surface 50 by printed circuittechniques. Output transducer 29 consists of interdigital fingers 30 and3] which are likewise affixed to surface 51 as shown. The spacingbetween interdigital fingers D, and D is set to tune the input andoutput transducer to the operating frequency of the limiter. In thepresent case D,=D,,=(A/2). The distance L between input and outputtransducers is determined by the surface wave power densitycharacteristics for the particular substrate member and operatingfrequency. Design and fabrication of the limiter comprehends applying anelectromagnetic wave input signal to input transducer 32 and measuringthe fundamental harmonic power densities of the resultant acousticsurface wave propagating along the surface 51. By way of example, thecurves 17, 18, 19 and 20 of FIG. 2 illustrate such power densities forthe fundamental and harmonics of a 905 MHz input signal. The outputtransducer is positioned at the point of maximum interaction between thefundamental and the harmonics. This substantially coincides with thefirst zero slope of fundamental curve 17. A limiter designed from theabove-referenced curves would therefore have a distance L betweentransducers of 4.75 mm. and transducer finger spacing D =D of 1.93 ,u.m.Referring now to FIG. 7, the curve 47 disclosed therein illustrates thelimiting action of a limiter so constructed. The limiting action of thelimiter is clearly evident from an examination of FIG. 7 since increasesin input power above a certain level does not result in an increase inoutput power. Since this limiting action occurs at a given acousticpower density it is only necessary to vary the lengths of theinterdigital transducer fingers in order to obtain limiting at anyspecific value of total electromagnetic input power level. The limits onthis process therefore are only: excess power dissipation in thetransducer fingers; and, excess diffraction losses. Within these limitsthis process can also be used to obtain efficient harmonic generation atany convenient value of input power.

Referring now to FIG. 5, there is disclosed thereby 1 microwave acousticsurface wave mixer that is fabricated and operates in accordance withthe principle of the invention. Substrate member 35 of piezoelectricmaterial has input transducer 37 and output transducer 36 affixed byprinted circuit means to its acoustic surface wave propagating surface52. Input transducer 37 comprises interdigital fingers 40 and 41 whichare spaced to be tuned to the operating frequency of the device. Inputtransducer 37 is also adapted to accept simultaneously twoelectromagnetic wave input signals of different frequencies. Outputtransducer 36 comprises interdigital fingers 38 and 39 that are spacedto be tuned with the sum or the difference signal depending upon theproposed use of the mixer. Output transducer 36 is positioned at adistance L from input transducer 37 determined by the peak of the powerdensity acoustic surface wave curve of the sum or difference signal.Referring to FIG. 8, there is illustrated such a curve which representsthe sum power density acoustic surface wave curve for electromagneticinput signals of 785 MHz. and 905 MHz. A mixer responsive to the sum ofthe inputs utilizing this curve would therefore have its input andoutput transducer spaced at approximately L=4.0 mm.

A microwave acoustic surface wave amplifier embodying the concepts ofthe present invention is illustrated by FIG. 6. The amplifier comprisespiezoelectric substrate member 42, electromagnetic wave to acousticsurface wave input transducer 43, modulating means 44 and demodulatingmeans 45. Substrate number 42 has one polished surface 53 adapted topermit propagation of acoustic surface waves therealong.

Input transducer 43 consists of interdigital fingers spaced to be intune with the operating frequency of the device and affixed to surface53 by printed or integrated circuit techniques. Modulating means 44 canbe any suitable means for modulating acoustic surface waves such as gapinteraction of carriers in a silicon film member placed in closeproximity to surface 53. Demodulating means 45 likewise can be anysuitable known means for demodulating acoustic surface waves. In orderto practice the present invention with regard to such an amplifier,acoustic surface wave power density curves must be taken to determinethe proper position of the input transducer, modulator and demodulatoron the substrate member. In this regard it is only necessary to take thepower density curve of the fundamental. Curve 17 of FIG. 2 is typical ofsuch a curve. In order to achieve amplification the modulating means 44is affixed to the substrate member 42 at a point coinciding with thefirst zero slope of the curve. This is approximately 5.0 mm. for curve17. The demodulating means 45 is then affixed to the substrate member ata point coinciding with the second zero slope of the curve. This in thepresent example is approximately 9 mm. from the input transducer. Anamplifier based on the curves of FIG. 2 therefore would have modulatorand demodulator spacings of L,=5 mm. and L =4 mm. It is apparent from anexamination of FIG. 2 that amplifier gain is represented by the negativeslope of the curve 17 between 5 mm. and 9 mm.

While the invention has been described in its preferred embodiments, itis understood that the words which have been used are words ofdescription rather than words of limitation and that changes within thepurview of the appended claims may be made without departing from thescope and spirit of the invention in its broader aspects.

What is claimed is:

I. A mixer comprising a substrate member of piezoelectric materialhaving a propagation surface adapted to permit the propagation ofacoustic surface waves therealong,

an electromagnetic wave to acoustic surface wave input transducerdisposed on said propagation surface,

means for delivering first and second electromagnetic wave signals ofdifferent frequencies to said input transducer or to two inputtransducers, and

an acoustic surface wave to electromagnetic wave output transducerdisposed on said propagation surface,

said output transducer being positioned at a point on said substratemember that substantially coincides with the maximum power densityoccurrence of the acoustic surface wave propagated from said inputtransducer in response to the sum or difierence frequency signal of saidfirst and second electromagnetic wave signals.

2. A mixer as defined in claim 1 wherein said input transducer comprisesinterdigital members spaced so as to be tuned with the mixer operatingfrequency and said output transducer comprises interdigital membersspaced so as to be tuned with said difference frequency.

3. The method of fabricating a mixer comprising the steps of renderingone surface of a piezoelectric substrate member suitable for thepropagation of acoustic surface waves, affixing an electromagnetic waveto acoustic surface wave input transducer to said surface,

applying a first electromagnetic wave input signal of a given frequencyto said input transducer,

applying a second electromagnetic wave input signal of a differentfrequency to said input transducer,

measuring and recording the power densities of the acoustic surface waveresponsive to the difference signal of said first and second inputsignals, and

affixing an acoustic surface wave to electromagnetic wave outputtransducer to said substrate member propagation surface at a pointsubstantially coinciding with the power density maximum of said acousticsurface wave.

4. A mixer comprising a substrate member of piezoelectric materialhaving a propagation surface adapted to permit the propagation ofacoustic surface waves therealong,

an electromagnetic wave to acoustic surface wave input transducerdisposed on said propagation surface,

means for delivering first and second electromagnetic wave signals ofdifferent frequencies to said input transducer, and

an acoustic surface wave to electromagnetic wave output transducerdisposed on said propagation surface,

said output transducer being positioned at a point on said substratemember that substantially coincides with the maximum power densityoccurrence of the acoustic surface wave propagated from said inputtransducer in response to the sum frequency signal of said first andsecond electromagnetic wave signals.

5. A mixer as defined in claim 4, wherein said input transducercomprises interdigital members spaced so as to be tuned with the mixeroperating frequency and said output transducer comprises interdigitalmembers spaced so as to be tuned with said sum frequency.

1. A mixer comprising a substrate member of piezoelectric materialhaving a propagation surface adapted to permit the propagation ofacouStic surface waves therealong, an electromagnetic wave to acousticsurface wave input transducer disposed on said propagation surface,means for delivering first and second electromagnetic wave signals ofdifferent frequencies to said input transducer or to two inputtransducers, and an acoustic surface wave to electromagnetic wave outputtransducer disposed on said propagation surface, said output transducerbeing positioned at a point on said substrate member that substantiallycoincides with the maximum power density occurrence of the acousticsurface wave propagated from said input transducer in response to thesum or difference frequency signal of said first and secondelectromagnetic wave signals.
 2. A mixer as defined in claim 1 whereinsaid input transducer comprises interdigital members spaced so as to betuned with the mixer operating frequency and said output transducercomprises interdigital members spaced so as to be tuned with saiddifference frequency.
 3. The method of fabricating a mixer comprisingthe steps of rendering one surface of a piezoelectric substrate membersuitable for the propagation of acoustic surface waves, affixing anelectromagnetic wave to acoustic surface wave input transducer to saidsurface, applying a first electromagnetic wave input signal of a givenfrequency to said input transducer, applying a second electromagneticwave input signal of a different frequency to said input transducer,measuring and recording the power densities of the acoustic surface waveresponsive to the difference signal of said first and second inputsignals, and affixing an acoustic surface wave to electromagnetic waveoutput transducer to said substrate member propagation surface at apoint substantially coinciding with the power density maximum of saidacoustic surface wave.
 4. A mixer comprising a substrate member ofpiezoelectric material having a propagation surface adapted to permitthe propagation of acoustic surface waves therealong, an electromagneticwave to acoustic surface wave input transducer disposed on saidpropagation surface, means for delivering first and secondelectromagnetic wave signals of different frequencies to said inputtransducer, and an acoustic surface wave to electromagnetic wave outputtransducer disposed on said propagation surface, said output transducerbeing positioned at a point on said substrate member that substantiallycoincides with the maximum power density occurrence of the acousticsurface wave propagated from said input transducer in response to thesum frequency signal of said first and second electromagnetic wavesignals.
 5. A mixer as defined in claim 4, wherein said input transducercomprises interdigital members spaced so as to be tuned with the mixeroperating frequency and said output transducer comprises interdigitalmembers spaced so as to be tuned with said sum frequency.